1. Field of the Invention
The invention in general relates to voltage probes that pass a test signal from an electronic circuit element to be tested to an oscilloscope or other electronic measurement device, and more particularly to a probe system that incorporates many individual probe channels in a small package, yet passes the test signal with high signal integrity over a high band width.
2. Statement of the Problem
Voltage probes are commonly used to pass analog test signals from a circuit under test to an oscilloscope or other electrical or electronic test instrument. Such an electronic probe must be capable of passing an electrical signal on a node or pin of the circuit under test to the test instrument without distorting it, i.e. with high signal integrity. Further, it should not apply any voltage or current to the circuit under test.
Present-day electronic circuits operate over frequencies from DC to several gigahertz. Thus, test probes capable of being used with a wide variety of circuits must be able to provide high signal integrity over a wide band width of frequencies.
The art of test probes that pass analog signals to a test instrument should be distinguished from the digital test equipment art. In the latter art high signal integrity is not a significant goal, since digital test instruments only need to detect the rise or fall of a digital signal.
Integrated and hybrid circuits are becoming both more complex and smaller, leading to ever higher numbers of package leads crowded into less and less space, that is, the leads are becoming extremely dense with very tight pitches. To deal with such a high number of leads, one solution, which is the subject of copending United States Patent application Serial No. (PDN 1094751), is to use a high number of multiplexed probe channels, each of which can probe a different lead of the circuit under test. The probes should be small so that many of them can be used on the same device under test. In a multiplexed probe system, all of the probe channels must come into close proximity in the multiplexer. This physical closeness leads to coupling between the probes. That is, some of the signal on one probe channel couples to adjacent probe channels. We have found that this coupling is particularly severe in the reverse direction at high frequencies. This is unacceptable, since the basic requirement of an analog probe is to pass the signal from the circuit under under test to the test instrument essentially unchanged, except for amplitude. Further, there will necessarily be some input capacitance in the probe amplifier circuit. Any input capacitance in the probe amplifier will cause reflections back up the input cable. Thus, the reverse coupling between adjacent probes that are physically close together and the signal reflection due to input capacitance of a probe amplifier, both produce reverse signals that propagate back up the input cable. In conventional probes, when the reverse signal reaches the probe tip it is re-reflected back down the cable and then appears at the amplifier input as an error which is a degradation to the probes accuracy and performance.